phases: mechanism to allow extension to alter initial computation of phase This commit add a whennodata list where extension can register a callback to be called if no phase related data are found in the repository. The goal is to ensure the existing extension that move phase data in 2.1 can compute consistent phase boundary for existing repo.

qclone: add a few comment and blank line This help readability.

mq: turn changeset draft on qfinish (except if qparent is secret) As mq automatically sets changesets as secret, it should make them draft when he is done with it. We do not move them automatically to draft when we detect that something else have also set them as secret through their parents.